Efficacy of the Combination of Eucalyptus globulus (Myrtaceae) and Cananga odorata (Annonaceae) Essential Oils against Laboratory-reared and Field-collected Larvae of Anopheles gambiae (Diptera: Culicidae)

Abstract

The persistent challenge of malaria, primarily transmitted by Anopheles gambiae in sub-Saharan Africa, has been exacerbated by increasing insecticide resistance among mosquito populations. This study evaluated the larvicidal activity of essential oils (EOs) from Eucalyptus globulus (Eg) and Cananga odorata (Co), and their binary mixtures, against laboratory-reared and field-collected third-stage larvae of An. gambiae s.l. over 24 hours. EOs were obtained by hydrodistillation at yields of 0.78% (Eg) and 0.57% (Co), and characterised by GC-MS. The EO of E. globulus was dominated by eucalyptol (34.25%) and α-pinene (26.20%), while C. odorata was characterised by linalool (20.45%), benzyl benzoate (13.47%) and benzyl acetate (13.10%). Larvicidal bioassays conducted by World Health Organization (WHO) protocols revealed that, against laboratory larvae, LC₅₀ values ranged from 52.30 ppm (75% Co + 25% Eg) to 82.22 ppm (pure Eg), with the 75% Co + 25% Eg mixture exhibiting synergistic interaction (co-toxicity index = 126). Against the field strain, LC₅₀ values were higher (86.42–166.87 ppm), yet the same mixture retained the highest efficacy (LC₅₀ = 86.42 ppm, CI = 149.15). Resistance ratios (RR₅₀) ranged from 1.65 to 2.25 across all formulations, all below the WHO threshold of 5, indicating susceptibility or low resistance. This study represents the first in-depth characterization of E. globulus and C. odorata binary mixtures specifically against An. gambiae, highlight the potential of C. odorata-dominant formulations as sustainable, eco-friendly botanical larvicides for malaria vector control in insecticide-resistant areas.